TECHNICAL FIELD
The invention relates to a process for the purification of an off gas (exhaust gas) which originates from a reaction process operated under a pressure of 5 to 50 bar and which is contaminated with aliphatic and/or aromatic substances, by scrubbing off the off gas. The off gas to be purified originates from an oxidation of xylene with an oxygen-containing gas, for example from the oxidation of para-xylene (p-X) with air by the so-called Witten DMT process for the preparation of dimethyl terephthalate (DMT).
In the Witten DMT process, a mixture of p-X and methyl para-toluate (p-TE) in the liquid phase is oxidized in the absence of solvents foreign to the process and of halogen compounds at a pressure of about 4 to 8 bar and a temperature of about 140.degree. to 180.degree. C. with air (atmospheric oxygen) in the presence of dissolved heavy metal oxidation catalysts, for example in the presence of a mixture of cobalt and manganese compounds (cf. German Patent 20 10 137).
In this process carried out industrially on a large scale, the oxidation with air produces an off gas which, depending on the oxidation pressure and on the temperature and the physical properties, is typically saturated with the following fractions:
a) 0.4 to 0.6% by weight of a relatively high-boiling fraction of DMT, monomethyl terephthalate (MMT), terephthalaldehydic acid methyl ester (TAE), para-toluic acid (p-TA) and terephthalic acid (TPA). Some of these products form during the cooling off sublimates having a high melting point, unless they are dissolved in the other components; PA1 b) 14 to 22% by weight of a water-insoluble fraction having a medium boiling point and consisting of products such as p-TE, methyl benzoate (BME) and p-X; PA1 c) 7 to 10% by weight of a fraction having a medium boiling point and consisting of water-soluble products such as acetic acid, formic acid, water of reaction and methanol; PA1 d) 0.2 to 0.3% by weight of low boilers, such as methyl acetate, methyl formate, acetaldehyde, formaldehyde, dimethyl ether and the like. PA1 e) the nitrogen of the air; PA1 f) a residual oxygen content of 0.5 to 3% by weight (of 6 to 8% by weight when the explosion limits are reached); PA1 g) the byproducts of the reaction: 1 to 3% by weight of CO.sub.2 and 0.3 to 2.0% by weight of CO.
The carrier off gas for the above substances consists of:
From the economic point of view, the above-mentioned substances are divided into so-called useful products and waste substances. The useful products are recycled to the total process.
The waste substances must be disposed off in a manner which is as environment-friendly as possible. These waste substances include CO, the low boilers acetaldehyde, formaldehyde and dimethyl ether, and acetic acid and formic acid.
In the case of the useful products, a distinction can be made between water-soluble ones, such as methanol, and water-insoluble ones, such as p-X, DMT, p-TE, etc.
Regarding the "Witten DMT process" (Katzschmann process), reference may be made to German Patent 1,041,945, German Patent 1,299,627, DE-A-20 14 012, DE-C3-22 44 662, DE-C3-24 27 875, DE-C2-29 16 197, DE-C2-30 11 858, DE-C2-30 37 045, DE-C2-30 44 617, EP-B1-0 053 241, DE-C1-31 51 383, EP-B1-0 156 133, EP-B1-0 157 122, DE-C1-34 40 650 and WO 90/09367 and Hydrocarbon Processing, November 1983, page 91. Where reference is made in the present description to the total process, these publications are referred to.
The off gas from the oxidation by the Witten DMT process is usually cooled in several stages, the relatively high-boiling and medium-boiling components substantially condensing stepwise. After scrubbing with cold water, the off gas stream is substantially freed from residual organic substituents in an activated carbon adsorption (Ullmann, 4th Edition 1982, Volume 22, page 529 et seq.; DE-C1-34 40 650 and DE-A1-24 61 092).
A similar waste gas purification with cooling and subsequent scrubbing with water is known for the oxidation waste gases in the so-called AMOCO process (Ullmann, 4th Edition 1982, Volume 22, page 519 et seq.).
This waste gas purification has the disadvantage that the adsorption stage (activated carbon plant) can only be operated discontinuously, that the working up of the activated carbon desorbate is expensive and that the energies contained in the waste gas can be utilized only to a limited extent. Finally, it is not possible to remove the CO present in the waste gas by means of adsorption using activated carbon.
GB-A-2 097 476 (=DE-A1-32 15 437) discloses a process for utilizing the mechanical energy of a residue gas, in which the residue gas is used for cooling a gas turbine with subsequent mixing the residue gas with the combustion air. Combustion of the residue gas is neither intended nor possible in this process.